With the growing interest in renewable energy, including the use of solar power, there is an increasing demand for more efficient solar cells. Solar cells or photovoltaic (PV) cells are devices that convert solar energy into electricity by the photovoltaic effect, and solar cells are widely used in devices ranging from satellites and other applications including portable consumer electronic devices that are remote from a conventional power source. More recently, large solar power collection systems with arrays of cells or PV modules are being used to supply power to electrical grids for distribution to consumers. Several concerns are limiting the implementation of solar cells, including cost of materials and manufacturing, environmental concerns with materials such as lead, and low efficiency of the cells. As a result, researchers continue to look for ways to lower manufacturing costs and designs that use more environmentally friendly materials. Further, existing solar cells such as those based on a silicon substrate typically have efficiencies of 10 to 20 percent, and, as a result, even small increases (e.g., of one to several percent) in efficiency represent large relative gains in being able to convert solar energy into useful electricity (e.g., an increase in efficiency of 1 to 2 percent represents a 5 to 20 percent or higher gain in efficiency for a cell design).
In many microelectronic fabrication processes, thick film conductors are widely used in the devices including silicon (Si) solar cells. For example, Si solar cells are presently fabricated with a contact formed by screen printing a metallic paste such as a mostly silver (Ag) paste upon an anti-reflective (AR) coating (e.g., SiNx) previously applied to a substrate. For Si solar cells, a glass frit with a low melting point is an essential component of these metallic pastes because the fits provide a good adhesion of the metal (e.g., Ag) to the semiconductor substrate and also serve as a “fire through” agent or burn-through agent. The burn-through agent represents a significant volume fraction of the screen print material. “Fire through” is the name given to the process of establishing an electrical contact between the metal (e.g., Ag) and the Si substrate by etching through the AR coating, which is typically made up of a dielectric material, or through a native oxide, at elevated temperatures. Additionally, the glass fits act to reduce the temperatures required to perform the metal-to-Si alloying to form an electrical contact in the solar cell.
The addition of glass in the Ag pastes helps to facilitate an electrical contact between the Ag of the deposited paste and the Si substrate through an AR layer or coating (e.g., a thin layer of SiNx AR coating). The fire through process typically takes several seconds and is typically performed at temperatures greater than 700° C., whereas without the addition of frits much higher temperatures (e.g., 850 to 1000° C.) along with longer annealing times (e.g., 10 to 20 minutes versus seconds) are required. Higher processing temperatures and longer anneal times are undesirable because high temperatures are often detrimental to the overall properties of the fabricated solar cells. Higher temperatures and longer processing times also increase production costs associated with Si-based solar cells and photovoltaics.
It is desirable to improve the process of fabricating the solar cell contact. The use of screen printing using Ag paste can be problematic in some applications because it is a contact deposition process that may cause damage to fragile and/or brittle substrates or components such as many Si solar cell structures. Another concern with the present methods of applying a silver contact is the effectiveness and/or size of the connection or contact between the silver and silicon of the cell substrate. With the use of glass fits in the Ag-containing paste, the effective contact achieved with the burn through process is sometimes only 1 to 3 percent of the area of the printed contact pattern. With such small area of the contact in order to achieve reasonable contact resistance very low contact resistivity in the areas of the contact is required. This is accomplished by doping Si to the levels above 1019 cm3 for the n-type emitter. High doping levels between the metal fingers of the front contact compromise blue response and reduce Voc of the cells. The compositions of the glass frits used in commercial pastes are typically kept proprietary but are known to generally contain a mixture of various oxides (e.g., PbO, SiO2, B2O3, and the like). The glass for the glass frits is formed by refluxing and homogenizing the oxide constituents at high temperatures (e.g., above 1000° C.), and then grinding and milling of the glass produces glass frit powder that is provided in the commercial paste. After the burn through or contact formation process is completed, a significant amount of the glass material remains in the contact, which is undesirable as it is a dielectric material and increases the resistivity of the contact. To address these inefficiencies, printing is often performed with lines or contacts that are wider and thicker than would be needed if better contact efficiencies and higher conductivity through the contact layer could be achieved, and Si solar cell manufacturing requires greater amounts of silver. In addition, current technical limitations of screen printing limit line size to 70 to 150 μm, which is much wider than needed. If line widths could be lowered to 40 to 50 μm then the overall efficiency could be improved just by limiting shadowing losses. Large size (1-5 μm) of the frit particles prevents the use of Ag pastes containing frits with higher-resolution deposition and patterning techniques such as inkjet printing.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.